Assume a Trans Replacement
If the truck looks a 'little' rough, and has a gooseneck hitch, and gauges, then I'd assume you will need to rebuild the trans. With the hitch and gauges the odds of it being a good trans are significantly reduced.
Unless of course, you can prove the trans has been upgraded or at least rebuilt sometime in the past.
If the auto trans is driven intellegently, with very little or no increase in power, AND it happens to be one of the factory transmissons that had tight clearances and seals that sealed, then you might get a few more miles out of the trans.
I've rebuilt and upgraded transmissions with 40K that were near trash, and also transmissions with 235K that still looked surprisingly good. It is a lot of luck whether the trans starts out as a good one or a lousy one from the factory.
You should drive a truck with an upgraded trans, then you will feel what a Cummins engine can do. A lot of power and performance is eaten up by the lousy stock Torque converter
Just negotiate the prce down to include a trans upgrade. If this is off of a dealer lot, emphasize the 'high' miles and the roughness of the truck and the gauges which 'must' mean that the truck was a hot-rod or run very hard. These are the same tactics that the dealers use to push down the price given on a trade-in, so use the same ammunition on them!!
Good luck, hope it works out.
Greg L. Tha Noise Nazi.